To pitch in, of my well over 100 forts (in the old 6.2 version).
~ Sapphire, Emerald, Ruby - fairly common. Extremely common to see these rare gems in SCHIST rock layers.
~ Diamonds - sporadic. mostly single clusters, with the rare multi-cluster. I cannot recall where I see these, but they are some what common (I can confirm almost every single world generates these).
~ Star Rubies, Yellow Diamonds, etc - I have NOT seen them a single time in 100+ forts in 6.2.
~ Pyrite and Amethyst seem to be the overwhelmingly abundant gem type I see (normally with 'simplified gems OFF').
~ Not relevant, but to note Malachite (Copper ore) is overwhelmingly the most abundant Ore as well, by up to 6x+ the rate of the next most abundant. I wonder if ores and gems 'compete' in some sense for spawning? Especially with gems who need specific 'stone veins' to grow in... *rubs head*
So I'm not sure if I've just had HORRID luck, or something really is busted and they aren't spawning.
EDIT:
~ After 40+ embarks (I prospect all in the actual map, before confirming if an evil cloud is thralling, etc, searching for a 'perfect map') and I have seen Diamonds, Sapphires, Emeralds, and Rubies galore. Not a single Star Sapphire, yellow Diamond, etc etc has appeared, anywhere. Will continue my hunt ad notes. Though, this version IS old, so when Meph gets the new one done I'll shift my focus to confirming there.